Very few Private Member’s Bills become law, but they stimulate debate on an issue, and this can lead to more powerful legislative efforts.

The BMA campaign is based on the premise that toxin levels produced by smoking in a closed vehicle are 23 times higher than those found in a smoky bar, and that children and the elderly are particularly at risk.

The response from ministers yesterday was cautious.

A Department of Health spokesman said: ‘We do not believe legislation is the most effective way to encourage people to change their behaviour.’

Instead Whitehall will launch a publicity campaign next spring warning of the dangers of smoking in cars and at home.

David Cameron told MPs that while he supported the ban on smoking in public places which has been in force since 2007, ‘I am much more nervous about going into what people do inside a vehicle’.

Backbench Tory MPs were scathing about the campaign for a ban.

Philip Davies, MP for Shipley, said: ‘It is a ridiculous idea, and typical of the health zealots of the BMA.

‘Why don’t they say what they really want, which is for smoking to be banned altogether? They don’t have the courage to say that, so they try to do it in small steps.’

Mr Davies added: ‘Banning smoking in cars would be a triumph for the nanny state. What people do in their own cars is their own business and the BMA should keep their noses out.’

Julian Brazier, MP for Canterbury, said: ‘The ban the BMA recommend would be both an intrusion into privacy and would waste a great deal of police time.

‘It would be one more example of the diversion of police away from their essential business of stopping real crime into a politically correct dead end.’

Mr Brazier added: ‘Where you smoke or not in your private car must be a personal choice.’

Civil liberties pressure groups were also critical. Sean Gabb of the Libertarian Alliance claimed that warnings of the risks of passive smoking were based on ‘junk statistics’.

'Junk statistics': Civil liberties groups have also been critical of the proposals

He added: ‘The demand for a smoking ban in cars is an instance of the “saving the kiddies” argument.

‘This proceeds by hiding the agenda of control behind a cloud of hot air about the need to protect children.’

However Mr Cunningham’s bill – to be presented in the Commons on Wednesday – is attracting support from MPs who believe publicity campaigns are not enough to curb smoking.

Liberal Democrat Stephen Williams, who chairs the all-party Parliamentary Group on Smoking and Health, said: ‘While welcoming the Department of Health’s commitment to launch a marketing campaign to encourage people to make their homes and cars smoke-free, having listened to the experts, I believe this will not be sufficient.

‘The killer fact for me was that just one cigarette smoked in a car during a 30-minute journey with the windows closed leads to levels of second-hand smoke about seven times that of the smoky bars that existed in this country before 2007.

‘This is clearly intolerable and it is time to turn the debate to how best to protect children from second-hand smoke.’